Perhaps you should try some spelling variations:
"operation theatre" » "operating theatre", "operating theatres", "operating theater"
"aberration theory" » "aberration test"
"operations theory" » "observations theory", "relations theory", "options theory"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("operator theory") OR ("operators theory"))) OR ((((("operation theatre") OR ("aberration theory"))) OR ("operations theory")))).
• Removing quotes may allow a broader search: (((( operator theory ) or ( operators theory ))) or ((((( operation theatre ) or ( aberration theory ))) or ( operations theory )))).
• Adding a wildcard symbol may retrieve word variants: (((("operator theory") or ("operators theory"))) or ((((("operation theatre") or ("aberration theory"))) or ("operations theory"))))*.